This invention relates to the polishing of a metal film, and, in particular, the invention relates to a method of polishing a metal film in a semiconductor device interconnection process.
In recent years, as semiconductor integrated circuits (referred to hereafter as LSI) have become more complex, new microtechniques have been developed. One of these is chemical mechanical polishing (referred to hereafter as CMP), which is often used in LSI manufacture, in particular for flattening of interlayer insulating films, for forming metal plugs and for inlay of interconnections in multi-layer interconnection processes.
This technology is disclosed, for example, in U.S. Pat. No. 4,944,836.
To achieve higher speeds in a LSI, attempts are being made to use a low resistance copper alloy, instead of the conventional aluminum alloys, as an interconnection material, however, with copper alloy, microprocessing by dry etching, which was used with aluminum alloy, is difficult. Therefore, the xe2x80x9cdamascenexe2x80x9d method is mainly employed, wherein an inlaid interconnection is formed by depositing a copper alloy thin film on an insulating film on which a groove is then formed by dry etching, and the copper alloy thin film is removed by CMP, except for the part inlaid in the groove. This technique is disclosed, for example, in Japanese Published Unexamined Patent Application No. 2-278822.
In general, slurries used for CMP of a copper alloy interconnection comprise a solid abrasive and an oxidizing substance as main components. The basic mechanism of CMP is to mechanically remove the oxide using a solid abrasive while oxidizing the surface of the metal by the oxidizing action of an oxidizing substance. This is disclosed on p. 299 of xe2x80x9cThe Science of CMPxe2x80x9d, edited by Masahiro Kashiwagi and published by Science Forum on Aug. 20, 1997 (in Japanese).
As solid abrasives, an alumina abrasive and silica abrasive with a particle diameter of several 10-several 100 nm have been used but most solid abrasives for metal polishing on the market are of the alumina type.
Generally, as oxidizing substances, hydrogen peroxide (H2O2), ferric nitrate (Fe(NO3)3) and potassium iodate (KIO3) are used, and these are described on p.299-p.300 of the aforementioned xe2x80x9cScience of CMPxe2x80x9d.
However, when interconnections and plugs were formed by CMP using a conventional slurry containing a solid abrasive for metal film polishing as a main component, the following problems (1)-(8) occurred.
(1) Denting (referred to hereafter as dishing) occurs wherein the surface of the central part of the metal interconnection inlaid in the groove formed in the insulating film is polished excessively compared to the periphery thereof, or a phenomenon (referred to hereafter as erosion) occurs wherein the insulating film surface around the interconnection is polished (FIGS. 5A, 5B).
The metal/insulating film selective ratio of a slurry intended for metal film polishing is as high as ten or more. This value is obtained by performing CMP on a wafer with only a flat metal film, and a wafer with only a flat insulating film, and comparing the polishing rates in the two cases.
However, it is known that when CMP is applied to a wafer where a metal film is deposited on an insulating film having a groove which is an interconnection pattern, excessive polishing occurs locally. This is due to the fact that there is unevenness on the surface of the metal film before CMP is performed, reflecting the groove which is the interconnection pattern. When CMP is applied, high pressure occurs locally according to the pattern density, and the polishing rate at these points is faster.
Therefore, dishing and erosion become conspicuous problems in pads of large area (are of about 0.1 mm side) or with crowded interconnection patterns. These problems are mentioned in J. Electrochem. Soc., p. 2842-2848. Vol. 141, No. 10, October 1994.
(2) Scratches (polishing marks) occur due to the solid abrasive used for polishing. In particular, alumina, which is the main material used as a metal polishing abrasive, has a greater hardness than silicon dioxide, which is the main material of the insulating film. Therefore, scratches occur on the surface of an insulating film exposed by CMP in addition to the surface of the metal film used for the interconnection. A slurry remains behind in the scratches on the insulating film surface, and this causes a malfunction of the semiconductor device due to heavy metal ion contamination. It also affects the shape of the upper layer interconnection, and causes short circuits. The scratches on the metal film surface cause poor continuity and deterioration of electromigration resistance.
In order to prevent scratches, the down force and the platen rotation speed are reduced when CMP is employed. However, it is difficult for even this method to prevent scratches in a soft metal, such as copper.
The scratches can be reduced by using a soft polishing pad, but dishing and erosion become more serious, and the flatness after CMP deteriorates. It was therefore suggested to perform CMP with a hard polishing pad in a first stage, and then to finish with a soft polishing pad, i.e. to perform a two-stage CMP. A new problem, however, arises in this case in that the throughput falls.
(3) Due to the high frictional force between the polishing abrasive and the metal film surface when CMP is performed, peeling occurs between the metal film and the lower insulation layer, or between the spin-on-glass (referred to hereafter as SOG) in the lower insulating layer and the chemical vapor deposition (referred to hereafter as CVD) oxide film. To prevent peeling, the down force and the platen rotation speed may be reduced, but if an attempt is made to completely prevent peeling, the CMP rate falls and the polishing time becomes longer, which is not practical. This can be resolved by using a soft polishing pad, but dishing and erosion become serious, and the flatness after CMP deteriorates.
(4) Since a large amount of polishing abrasive remains behind on the wafer surface after CMP, cleaning must be performed before applying the next step, and foreign matter must be removed until the amount thereof is below a specified level (e.g., there must be no more than 100 particles of foreign matter greater than 0.2 xcexcm in one wafer). A cleaning machine which employs mechanical cleaning together with chemical cleaning is needed for this purpose.
The cleaning technique is very complicated, as shown in FIG. 11. Brush-cleaning and megasonic cleaning that use a reagent fluid together are mainly used. The brush materials must be special materials which do not damage the metal film surface, and, for example, ammonium hydroxide or an aqueous solution of hydrofluoric acid are used as a reagent fluid.
Megasonic cleaning is a cleaning method using a high frequency of 800 kHz applied to the cleaning fluid so as to remove abrasive material from the substrate. This cleaning is more powerful than conventional cleaning by ultrasonic waves (40 kHz). In this technique, sufficient energy or force must be supplied to remove the abrasive material from the substrate. On the other hand, the output must be set in a range that does not damage the metal film and the insulating film. An example of post-CMP cleaning is disclosed on p. 172 of the May 1995 edition of Semiconductor World (in Japanese).
(5) Consumable items used for CMP are costly. This is because the production cost of abrasives used in the slurry is high, and great care must be taken to adjust the particle size. In particular, alumina abrasive is several times higher in price compared with silica abrasive.
In general, foaming polyurethane is used as a polishing pad. When CMP is performed, the polishing abrasive adheres to this polishing pad, clogging occurs, and the CMP rate drops.
To prevent this, the polishing pad surface needs to be sharpened with a whetstone (referred to hereafter as a conditioner) to which diamond particles were made to adhere. Therefore, the life of the polishing pad is short, and it represents a high cost consumable item next to the rest of the polishing abrasive. The cost of the CMP process is discussed in xe2x80x9cRecent Trends and Problems in CMP Apparatus and Related Materials, Realize Inc., New Tech Lecture, May 1996.
(6) Regarding CMP-related machines and equipment, in addition to the above-mentioned CMP machine and post-cleaning machine, a slurry feeder and a processor of waste fluid containing slurry are required. Therefore, the cost of the whole CMP facility becomes very high. A stirrer is also needed to prevent sedimentation of abrasive in the slurry feeder, and equipment was required to keep a slurry circulating through the piping to stop it from depositing. The cost of waste fluid processing is also high, and a recycling technique is needed.
(7) It is also a problem that the throughput of the whole CMP process is low. In a CMP facility, it is usual to condition the polishing pad, perform a first CMP operation to polish the metal film, and perform a second CMP operation (buff polishing) to remove the damaged layer of insulating film exposed by the first CMP operation. As the post-cleaning machine involves brush cleaning, wafers are usually cleaned wafer by wafer. Therefore, the throughput of the whole CMP process represents the lowest throughput in the semiconductor device manufacturing process. An example of the overall CMP process is given in detail, for example, in the May 1995 edition of Semiconductor World, p. 172.
(8) Although the CMP machine uses large amounts of the polishing abrasive, which tends to generate dust, it must be operated in a clean room. A system must be provided to suppress dust in the exhaust duct of the CMP machine, and a special room must be set up in the clean room to maintain the degree of cleanliness, which is costly.
All of the above problems are caused by performing CMP using a slurry containing a highly concentrated polishing abrasive. However, in one known CMP method, to increase the polishing rate, the surface of the metal is oxidized by an oxidizer, and the surface of metal that was exposed by mechanically removing this oxide layer with a polishing abrasive is re-oxidized. This process of oxide layer formation and mechanical removal is repeated. In other words, the polishing abrasive was necessary to provide a mechanical removal effect, whereby the oxide film could be rapidly removed, and when the polishing abrasive was not added, a practical CMP rate was not reached.
In Japanese Published Unexamined Patent Application No. 7-233485, a comparison example is given where CMP was performed with a polishing solution to which a polishing abrasive was not added (0.1 wt % aminoacetic acid and 13 wt % hydrogen peroxide). It is reported that in this case, the polishing rate was 10 nm/min, about 1/10 of that of a polishing solution to which an alumina polishing abrasive was added and about 2/7 of that to which a silica polishing abrasive was added.
FIG. 2 is the result of an additional test based on said Japanese Published Unexamined Patent Application No. 7-233485. This test measured the hydrogen peroxide aqueous concentration dependency of CMP rate and etching rate in a polishing solution containing 0.1 wt % aminoacetic acid and hydrogen peroxide (not containing abrasive), so as to reproduce the results of the aforesaid Koho. It should be noted that FIG. 2 shows a concentration of 30% aqueous hydrogen peroxide, and so to make a comparison with the above Koho, the results should be multiplied by a factor of 0.3. The hard pad IC1000 of the Rodel company was used as a polishing pad. The rotation speeds of the platen (diameter: 340 mm) and holder were both 60 rpm, and the down force was 220 g/cm2 (same as CMP condition of this invention). From the result of FIG. 2, it is seen that when an abrasive is not included, the CMP rate is barely 20 nm/min, i.e. a practical CMP rate is not obtained. When the hydrogen peroxide concentration is low, the etching rate is fast, and the stability of polishing becomes poor. The stability rises if the hydrogen peroxide concentration is increased, but the CMP rate becomes very low, which is disadvantageous from the viewpoint of throughput.
On further examination, it was also found that the still solution etching rate (the etching rate in the case when a stationary sample was immersed in a polishing solution which was not stirred) does not fall exactly to zero even at a high hydrogen peroxide concentration. When the polishing solution is stirred, and the etching rate is measured (the etching rate in a stirred solution is near to the etching rate during CMP), it is seen that the etching rate increases, and exceeds xc2xd of the polishing rate.
Therefore, it was found that unless the CMP rate was increased by including an abrasive and the ratio of the CMP rate and stirred etching rate (referred to hereafter as rate ratio) was increased, the solution could not be used as a polishing solution. When the rate ratio is low, etching proceeds in depressions not in contact with the polishing surface, and flatness is lost. In fact, using a polishing solution wherein the hydrogen peroxide solution concentration was varied, it was found that a polishing time of from 40 minutes to 1 hour and 30 minutes was needed.
A cross-section of the copper interconnection formed is shown in FIGS. 22A, 22B. Most of the copper which would have been left in the groove of the silicon dioxide film was etched out. As a result of a continuity test using a meandering pattern (line width 0.3-3 xcexcm, length 40 mm), the yield was 0%. Therefore, this could not be used as an LSI interconnection. This is due to the fact that as the CMP rate was slow, etching occurred during a long polishing time.
If the concentration of aminoacetic acid is raised, the CMP rate increases, but the stirred etching rate also increases and the same result is obtained. It was found that to suppress etching, potassium hydroxide may be added to the polishing solution to adjust the alkalinity to pH10.5. However, a problem occurs in that the selective ratio falls and erosion occurs due to the etching of the silicon dioxide film by potassium hydroxide. Potassium ions which remain behind spread through the insulating film, and cause deterioration of the characteristics of the semiconductor device.
This problem is due to the fact that aminoacetic acid itself has not much ability to make copper oxide water-soluble. As seen from the pH-oxidation/reduction potential diagram on p.387 in M. Pourbaix, xe2x80x9cAtlas of Electrochemical Equilibria in Aqueous Solutionsxe2x80x9d, 1975, published by NACE, and shown in FIG. 9, the range in which copper is made water-soluble as a copper ion (domain of corrosion) is pH7 and below, and as aminoacetic acid is neutral, its effect is weak.
FIG. 26 shows the difference of the corrosion rate (etching rate) in the domain of corrosion and the domain of passivation of copper. The solid line shows the corrosion rate when the oxidation-reduction potential is the same for the citric acid-based polishing solution and the aminoacetic acid-based polishing solution in FIG. 9. As typical examples, corrosion rate was plotted for a polishing solution comprising a mixture of citric acid and aqueous hydrogen peroxide in the domain of corrosion, and a polishing solution comprising a mixture of aminoacetic acid and aqueous hydrogen peroxide in the domain of passivation. Both polishing solutions were prepared with equal mole ratios. Hence, in the domain of corrosion, copper is rendered water-soluble and ionized at a much faster rate than in the domain of passivation.
This is mentioned in Proceedings of the CMP-MIC Conference, 1996, p. 123. Actually, it is reported that aminoacetic acid has no ability to etch copper oxide, but if copper oxide cannot be made water-soluble, it remains on the insulating film which is exposed after performing CMP, and causes electrical short circuits between interconnections. If the slurry contains an abrasive, the copper oxide is easily removed by mechanical action.
Conventional metal etching solutions lie within the above-mentioned domain of corrosion, but it is not certain that they can all be used as CMP polishing solutions for LSI multi-layer interconnections. This is because a slow etching rate is suitable for CMP polishing solutions. This is described, for example, in relation to abrasion experiments on copper surfaces using an aqueous solution of nitric acid, in the Journal of Abrasive Polishing, p. 231-233, Vol. 41, No. 1, 1997 (in Japanese). It is reported that when there is no abrasive, the CMP rate is low, but due to the absence of scratches, the solution is suitable as a polishing solution. However, the etching rate of this polishing solution was not studied, and there was no attempt to form an interconnection structure. As a result of performing additional tests on this polishing solution, it was found that the still solution etching rate of copper using 1 vol % aqueous nitric acid is 50 nm/min, but a sufficiently large ratio could not be obtained for the CMP rate of 80 nm/min mentioned in the aforesaid Journal. Further, when CMP was applied to form an inlaid interconnection, the copper in the part which should have formed the interconnection was etched and almost completely lost. Hence, polishing can be performed with a polishing solution wherein the etching rate is not suppressed, but an inlaid interconnection cannot be formed.
This invention, which was conceived in view of the aforesaid problems, therefore aims to provide a polishing method and semiconductor manufacturing method which permit at least one of the following to be attained: (1) control of dishing and erosion in the formation of an inlaid interconnection, (2) reduction of scratches, (3) reduction of peeling, (4) simplification of post-CMP cleaning, (5) cost reduction of polishing solutions and polishing pads, (6) simplification of slurry supply/processing equipment, (7) higher throughput, and (8) less dust.
The above objects are attained by a metal film polishing method wherein a metal film surface is mechanically rubbed using a polishing solution, which does not comprises a polishing abrasive or comprises a polishing abrasive at a low concentration of less than 1 wt %, and which has a pH and oxidation-reduction potential within the domain of corrosion of the metal film. A substance for suppressing corrosion (an inhibitor) may be added to the polishing solution as necessary.
The above objects are achieved by mechanically rubbing the metal film surface with a polishing solution 1 comprising an oxidizer (substance which removes metal electrons and raises the atomic valence) and a substance which renders oxides water-soluble. In this case, it may be applied to a metal film of Cu, W, Ti, TiN or Al.
The above objects are achieved by mechanically rubbing the metal film surface with a polishing solution 2 comprising a substance which renders the aforesaid metal water-soluble. In this case, it may be applied to metal films of Al or the like, which are commonly metals having a lower ionization tendency than hydrogen. Examples of substances which render the metal water-soluble are hydrochloric acid, organic acids, or alkalis such as ammonium hydroxide. The above objects are also achieved by using ammonium hydroxide as the substance which renders the metal water-soluble in the case of copper, which has a higher ionization tendency than hydrogen.
As the pH and oxidation-reduction potential of the above polishing solution are within the domain of corrosion of the metal, the metal can be rendered water-soluble, and metal remaining on the surface of the insulating film exposed on the surface of the polished substrate can be reduced. The domain of corrosion of each metal is given in the pH-oxidation/reduction potential diagram of Pourbaix mentioned above. For example, in the case of copper, Cu dissolves as a Cu2+ ion, if the pH less than 7 and the oxidation reduction potential is  greater than 0.2, as shown in FIG. 9. Otherwise, it dissolves as a CuO22xe2x88x92 ion in the alkaline region of pH greater than 12.5. Therefore, when polishing copper, it is desirable that it is in either domain of corrosion.
The Pourbaix diagram relates to an H2O system, and when other reagents are contained in the polishing solution, the range of the domain of corrosion in the pH and oxidation-reduction potential diagram will vary. The domain of corrosion in the context of this invention is defined by whether or not certain substances, including these substances, are within the range of pH and oxidation-reduction potential in which the polishing solution corrodes the metal. When the polishing solution contains both a corrosive substance and an inhibitor, the former is within the domain of corrosion shown by this invention.
When CMP is performed with the polishing solution 1 containing the aforesaid substance, the metal surface is first oxidized by the oxidizer, and a thin oxide layer is formed on the surface. Next, when a substance is supplied to make the oxide water-soluble, the oxide layer becomes an aqueous solution, and the thickness of the oxide layer decreases. The part of the oxide layer which became thinner is again exposed to the oxidizer, and the thickness of the oxide layer increases. This reaction is repeated as the CMP progresses. As projections 50 on the metal surface shown in FIG. 4A are constantly being mechanically rubbed by the polishing pad, reaction products on the surface are easily removed, the reaction is promoted due to local heating, the above oxidation/water-solubilization cycle is repeated, and the reaction progresses more rapidly than in depressions 49. Therefore, the polishing rate of the projections 50 is accelerated and they are flattened.
The inhibitor adheres to the metal surface, suppresses the reaction of the depressions and has the final effect of improving flatness. If the polishing solution is within the domain of corrosion of the Pourbaix diagram even when an inhibitor is added, the aforesaid reaction proceeds at the projections on the metal surface where the inhibitor has been removed by rubbing with the polishing pad, and the surface becomes flat. In other words, the polishing solution has both a corrosive effect and an inhibitory effect, and it is important to control both effects during CMP. The addition concentration of the inhibitor to the polishing solution should be such that inhibitor adhering to the projections on the metal surface is removed by the mechanical friction of the polishing pad. As a guide for this addition concentration, it is desirable that the CMP rate is maintained at 50 nm/min or more, and that the stirred etching rate should be several nm/min or less (rate ratio of the order of 50). When the inhibitor is added in greater concentration, the CMP rate may fall. If the CMP rate is sufficiently high without the addition of additives, and the etching rate is no greater than several nm/min, the substrate can be polished to a high degree of flatness even if the inhibitor is not added.
In the prior art CMP method, the metal surface was oxidized by an oxidizer, and the CMP rate was increased by mechanically removing this oxide layer using a polishing abrasive. According to this invention, however, although the concentration of polishing abrasive is reduced, a practical CMP rate is effectively obtained by the mechanical friction of the polishing pad alone by adding a substance which renders the oxide water-soluble.
The above-mentioned objectives (1)-(8) are achieved by the following polishing abrasive concentration ranges.
(1) The object of suppressing dishing and erosion is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.05 wt %.
(2) The object of reducing scratches on the insulating film surface is achieved by making the concentration of the above-mentioned polishing abrasive less than 1 wt %.
(2) The object of reducing scratches on the metal film surface is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.1 wt %.
(3) The object of reducing peeling is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.5 wt %.
(4) The object of improving cleaning performance is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.01 wt %.
(5) The object of reducing the cost of the polishing solution and polishing pad is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.001 wt %.
(6) The object of resolving problems of slurry supply and processing equipment is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.0001 wt %.
(7) The object of improving throughput is achieved by making the concentration of the above-mentioned polishing abrasive equal to or less than 0.01 wt %.
(8) The object of suppressing dust is achieved by not adding the above-mentioned polishing abrasive.
As hydrogen peroxide does not contain metal components and is not a strong acid, it is the most desirable oxidizer. Ferric nitrate and potassium iodate contain a metal component, but they have the effect of increasing the CMP rate due to their strong oxidizing power.
Examples of substances that make the aforesaid oxide water-soluble are acids, which convert the metal to a metal ion (e.g. Cu2+ ion). Typical inorganic acids are nitric acid, sulfuric acid and hydrochloric acid.
Organic acids or their salts have low toxicity and are easy to handle as a polishing solution. Typical examples are hydroxy acid and carboxylic acid such as citric acid, malic acid, malonic acid, succinic acid, phthalic acid, dihydroxysuccinic acid, lactic acid, maleic acid, fumaric acid, pimelic acid, adipic acid, glutaric acid, oxalic acid, salicylic acid, glycolic acid, benzoic acid, formic acid, acetic acid, propionic acid, butyric acid and valeric acid and their salts. Salts increase solubility, and preferably comprise ammonium salts, which do not contain a metal component, or an element which does not have an adverse effect on semiconductor elements (e.g. aluminum).
Of the aforesaid acids, citric acid, malic acid, malonic acid, succinic acid, dihydroxysuccinic acid and formic acid are preferable as the acid used in the polishing solution of this invention, due to the high CMP rate and the low etching rate achieved with their use.
Of the aforesaid acids, especially, citric acid and malic acid are generally used as food additives, and due to their low toxicity, low effluent problem, lack of odor and high solubility in water, they are preferable as the acid used in the polishing solution of this invention.
Due to its low solubility in water, it is preferable to use the salt of phthalic acid, but even if the pH changes when it is used as a salt, it is necessary to maintain the polishing solution in the domain of corrosion of the metal.
For example, when phthalic acid is used as a polishing solution for copper, if the hydrogenphthalate salt is used wherein only one of the two carboxyl groups in the phthalic acid molecule is substituted, the solubility in water increases and the pH can be kept on the acid side (domain of corrosion), so that it is suitable as a polishing solution.
In a phthalic acid salt where two carboxyl groups are substituted, the polishing solution is almost neutral, and the CMP rate decreases. The same is true of other organic acids.
One substance alone may be used both as an oxidizer and as a reagent for rendering the oxide soluble, for example, nitric acid which dissolves copper. This makes it possible to reduce the amount of reagent added, so the time and cost required to prepare the polishing solution can be reduced. Other oxidizers, such as hydrogen peroxide, can also be mixed in to increase the oxidizing power.
Moreover, the substance which renders the oxide water-soluble may be ammonium hydroxide, ammonium nitrate or ammonium chloride.
As described above, when an ammonium ion is contained in the polishing solution, the domain of corrosion changes, and copper is dissolved as a Cu(NH3)2+ ion even at a pH greater than 4.5. The pH-oxidation-reduction potential diagram for the Cuxe2x80x94NH3xe2x80x94H2O system is described in, for example, J. Electrochem. Soc., p. 2381, Vol. 142, No. 7, July 1995.
Examples of substances which suppress etching or oxidation are inhibitors and surfactants. Such a substance may be a material which, when mixed with the polishing solution, suppresses etching while permitting a sufficient CMP rate to be obtained. In particular, the most effective inhibitor for copper alloy is benzotriazole (referred to hereafter as BTA). Other substances with an inhibitory effect are tolyltriazole (referred to hereafter as TTA). BTA derivatives, such as BTA carboxylic acids (referred to hereafter as BTAxe2x80x94COOH), cystine, haloacetic acids, glucose and dodecyl mercaptan.
The effective surfactants are polyacrylic acid, polyammoniumacrylate, polymethacrylic acid, polyammoniummethacrylate. The most effective surfactant is polyammoniumacrylate due to the high CMP rate and low etching rate achieved with its use.
As a means of applying mechanical friction, a polishing pad may be used wherein more than 1 wt % of polishing abrasive is not supplied to the polishing solution.
The optimum hardness of the polishing pad is different depending on the object on which CMP is performed, but if, for example, a copper electrode pattern of 0.1 mm on a side is to be formed, and the permissible amount of dishing is 100 nm or less, it is desirable that when the polishing pad is pushed into a 0.1 mm opening under the load for performing CMP, the amount by which the polishing pad is compressed and pushed out from the opening is no more than 100 nm. A hard polishing pad meets this condition, and by using such a pad, dishing can be suppressed.
The damascene method is a technique wherein a metal film is formed on an insulating film which has an opening is polished, and the metal film is left in the opening.
However, when a plug of one xcexcm or less is to be formed, a soft polishing pad can also be used. Preferably, the polishing pad is hard, provided that scratches or peeling is not caused, however, it must be sufficiently soft to be able to follow projections on the substrate surface other than the interconnection or plug pattern, such as, for example, the curve of a wafer.
As in the case of the abrasive concentration of the polishing solution, the upper limit of polishing abrasive supplied from the polishing pad is different according to the above objects (1)-(8). For example, the object (1) of suppressing dishing and erosion is achieved by an abrasive concentration of 0.05 wt % or less.
A polishing solution having a CMP rate of 10 nm/min or less requires 80 min for performing CMP on a metal film of, for example, 800 nm. Therefore, it is not practical for preparing an interconnection structure, and as it does not resolve the above problems of throughput and cost, and so it is not defined as a polishing solution according to this invention.
It is preferable that the ratio of CMP rate and etching rate is 5 or more, and, if possible, is 10 or more. If it is less than this, the interconnection structure cannot be formed with high precision due to the etching effect produced in CMP even if the CMP rate is high. Preferably, the etching rate is no higher than several nm/min.
This invention is most effective for performing CMP on copper alloy or aluminum alloy wherein scratches, dishing and erosion may easily occur. It is also effective for reducing scratches on insulating films in other metal CMP, e.g. tungsten and tungsten alloy, and titanium and titanium alloy (particularly titanium nitride).